Toners used in electrophotographic copiers and printers are typically mixtures containing a plurality of substances such as a binder resin as a main component, a colorant, a release agent, and a charge controlling agent. Pulverized toners having a desired particle diameter can be obtained by melting and kneading these raw materials and pulverizing and classifying the resulting melted-kneaded product. Among the raw materials of the toners, the binder resin typically accounts for 80% by mass to 90% by mass and occupies a most part of the toners.
In recent years, it has become common to use polyester resins as binder resins of toners from the viewpoints of reducing power consumption in the copiers and reducing environmental impacts, because polyester resins can make toners easily fixable even at a low fixing temperature. Polyester resins for toners typically contain a bisphenol A skeleton in an alcohol monomer constituting the resins. Therefore, these polyester resins have excellent low-temperature fixability and also a sharp melting property that imparts excellent gloss level to the polyester resins. These properties make the polyester resins particularly suitable for full-color toners. However, there is a problem that the bisphenol A skeleton makes the polyester resins poorly pulverizable during toner production. Furthermore, although the polyester resins for toners have excellent low-temperature fixability, the polyester resins for toners often cause so-called copy blocking of printed sheets of paper being stuck to each other when the printed sheets ejected continuously from copiers form a large stack of sheets as is often the case in the fields of high-speed machines. Therefore, improvement of blocking resistance is a major issue.
Hence, there have been proposed polyester resins suppressed in bisphenol A skeletons in the resins in order to satisfy both of low-temperature fixability and pulverizability. PTL 1 proposes a toner resin and a toner composition that are made of a polyester resin obtained by polycondensing an alcohol component and a carboxylic acid component.
In this polyester resin, 80 mol % to 100 mol % of the carboxylic acid component is at least one of a terephthalic acid, an isophthalic acid, and lower alkyl esters thereof (with 1 to 4 carbon atoms on an alkyl group), 20 mol % to 100 mol % of the alcohol component is an aliphatic diol (85 mol % to 100 mol % of the aliphatic diol being 1,2-propylene glycol), and 0.1 mol % to 20 mol % of the total amount of the alcohol component and the carboxylic acid component is at least one of a trihydric or higher multihydric alcohol and a trivalent or higher carboxylic acid.
The invention of PTL 1 uses an aliphatic alcohol as an alcohol component in order to suppress bisphenol A skeletons in the alcohol component. This method can improve low-temperature fixability and pulverizability. However, as a result of suppressing bisphenol A skeletons, this method may degrade stress resistance (durability) so that the toner may be deteriorated due to stress of being stirred in a developing device, etc., and may also degrade filming resistance so that the toner may contaminate members such as a photoconductor. That is, this method is insufficient for satisfying stress resistance, filming resistance, and blocking resistance at the same time as satisfying fixability and pulverizability.